So, are you strengthening and stretching them? That would be my first instinct.
Pre-exhausting is usually a good way to go. Also, I'd consider trying totally new exercises. Maybe your pecs respond better to declines, or decline DBs, or weighted push-ups, or dips, or Swiss ball 1-arm DB pressing. Experiment.
And switch up your pre-exhaust movement. DB flyes of all angles (an incline flye followed by a decline barbell press is always fun), pec-decks, 1-arm pec-deck, cable crossovers, pullovers, play around until you find something that clicks.
Alot of people (not me, but what do I matter) also seem to prefer working heavy with partial movements, just the middle few inches of the press. Not fully locking out, but not coming all the way down. It could be worth trying.